1. Field of the Invention
This invention relates generally to the treatment of wastewater and wastewater sludges, and is more particularly concerned with anaerobic treatment wherein the sludge is conditioned and recycled to the anaerobic reactor, or directed to dewatering and drying.
2. Description of the Prior Art
Anaerobic treatment of wastewater and wastewater sludges is well known in the art. In the past this technology was used mainly for sludge digestion and for simplified treatment of small wastewater streams in septic tanks. Recently, the anaerobic method has been applied to treat larger flows of a more concentrated industrial wastewater, primarily in the food and beverage industries. These more recent applications have revealed general advantages and disadvantages of anaerobic treatment methods. Additionally, fundamental research has been conducted on treatment of more complex wastewater, including industrial wastewater samples and imitations thereof with poorly degradable and toxic organics. This research demonstrated additional capabilities, advantages and problems associated with anaerobic processes. The present status of anaerobic treatment technologies is very thoroughly described in a recent book, Design of Anaerobic Processes for the Treatment of Industrial and Municipal Wastes, edited by J. F. Malina and F. G. Pohland, Technomic Publishing Inc., 1992. Additionally, in 1992-1993 the applicant conducted a study of anaerobic treatment of a complex wastewater, which is used in this application to demonstrate advantages of the new and improved method.
Two major anaerobic treatment methods were developed in the past: (1) attached growth processes; and, (2) suspended growth processes. Some modifications are classified as hybrids of these methods. Advantages and disadvantages of prior methods are given in the above mentioned book. The major advantages of anaerobic systems are the low energy requirements, with potentially a net generation of energy, and a relative simplicity of treatment units and operations. Disadvantages of prior anaerobic treatment systems are summarized as follows:
1. Only wastewater with simple soluble substrate (easily degradable nontoxic constituents) can be adequately treated anaerobically. PA1 2. Suspended solids in the wastewater influent are not satisfactorily degraded unless retention time in the reactor is very long. PA1 3. Slowly and poorly degradable, or toxic, soluble constituents of the wastewater influent are not degraded unless retention time in the reactor is very long, or a bed of granular activated carbon (GAC) is provided. In the latter case, a portion of the GAC bed must be periodically replaced due to the accumulation of nondegraded adsorbed material. PA1 4. Liquid in anaerobic reactors often turns acidic due to the accumulation of fatty acids. This can be caused by an overloading with organics, or by a toxic effect of specific constituents in the feed. Accumulation of fatty acids and the respective drop in pH cause depletion in the methanogenic population. Further accumulation of fatty acids may cause suppression in the growth of acidogens. Inadequate growth of either group of organisms results in a process upset. Since methanogenic organisms have very slow growth rate, the anaerobic process recovery takes a long time. This problem becomes especially difficult during start-up operations because acidity control requires large quantities of alkalies, and the start-up process may last many months, and sometimes a year or longer. PA1 5. Toxic discharges (for example, slugs of acidic or alkalinic wastewater, or wastewater having elevated concentrations of toxic constituents) can poison the entire sludge population in the reactor, thus requiring a long restarting time. PA1 6. Two temperature regimes are used in anaerobic processes: thermophilic (about 55.degree. C.) and mesophilic (about 33.degree. C.). At temperatures lower than mesophilic, the process rate becomes very slow. PA1 7. Anaerobic processes are not intended for controlling nutrients and heavy metals. PA1 8. Anaerobic processes generate odorous gases such as hydrogen sulfide, and volatile organics. Accordingly, gases need to be collected even at small treatment plants, and are usually combusted. PA1 9. Anaerobic reactors for wastewater treatment have deficient systems for water distribution, gas collection, and sludge separation. Foam and scum often are accumulated in the upper sections of anaerobic reactors. PA1 10. Anaerobic reactors require a large area, because structural and cost considerations limit the total reactor height to 6 to 9 meters. PA1 1) Sludge is bioheated before bioflotation, which increases the process rate and insures bioflotation in colder climates. Bioflotation is achieved by subjecting the sludge to anaerobic conditions wherein methane, carbon dioxide, and/or nitrogen are preferably formed. In some instances, hydrogen sulfide will also be formed. These gases float up the sludge. PA1 2) Nitrates and nitrites are added to and mixed with the aerobically bioheated sludge, or immediately after the bioheating step in order to promote, respectively, denitrification and sludge flotation. The addition of nitrates accelerates the process described in previous paragraphs. PA1 3) The sludge flow is split into parallel aerobic bioheating and anaerobic digestion steps. In the anaerobic step, a long sludge age is maintained to cultivate acidogenic, methanogenic and denitrifying bacteria. Methanogenic and denitrifying bacteria consume fatty acids generated by the acidogenic organisms. At a longer sludge age, methanogenic and denitrifying bacteria deplete the fatty acids and other organic sources required for the growth of sulfate reducing bacteria. Accordingly, the growth of sulfur reducers is suppressed, the hydrogen sulfide is generated in very small quantities, and the process can be kept substantially odor free.
In summary, the above mentioned problems numbered 1 to 7 are related to a deficient sludge management strategy in prior art anaerobic wastewater treatment systems, and problems numbered 8 to 10 are related to deficient designs of anaerobic reactors. These two fundamental deficiencies limit the use of anaerobic treatment systems and cause operational problems in many of the systems already built.
Sludges generated in wastewater treatment processes, for example in biofiltration or activated sludge process, are usually directed for either aerobic or anaerobic biological stabilization. Sludge thickening may precede biological stabilization. Methods of sludge thickening include: gravity thickening in tanks designed as settling tanks, sometimes with gentle mixing; pressure air flotation; thermal gravity thickening/flotation thickening; vibratory filters; drum screens; and centrifuges.
During biological stabilization, sludge is substantially mineralized and becomes nonrotting; however, it retains a large proportion of water, which makes sludge disposal difficult. Accordingly, sludges are usually dewatered and dried, which may be accomplished on drying beds-the method preferred at smaller plants. Separate dewatering and drying are used at larger plants, the methods including vacuum filtration, filter pressing, centrifugation, etc. Separate methods of drying include drying beds, rotary drums, fluidized bed dryers, dryers with opposite jets, etc. Sometimes sludges are thickened, dewatered and dried without biological stabilization, or a chemical stabilization is used instead.
Thermal gravity thickening, and thermal gravity/flotation thickening show significant advantages over other thickening methods. These methods are described in the book Utilization of Wastewater sludges, by A. Z. Evilevich and M. A. Evilevich, Publishing House Stroyizdat, Leningrad (S. Peterburgh), 1988 (in Russian) and in Soviet Certificates of Invention Nos.: 300420, 1971; 381612, 1973; 1118623, 1984. Advantages include more rapid and more efficient separation (thickening) of sludge particles from water. A major disadvantage of these methods is in that heating of the sludge prior to the separation is done by a heat carrier, for example steam, which requires additional complex equipment, heat exchangers or the like, and energy from external sources (such as fuel). Sometimes flotation is not stable and portions of the sludge hang up in the mid depth or settle to the bottom of the flotation tank. Additionally, odor due to generation of hydrogen sulfide often occurs.